Isotope Fractionation

During the assimilation of the substrates vital to sustain the life of organisms, like carbon dioxide, nitrate and sulfate, the lighter isotopes are preferentially utilised. This isotopic fractionation is attributable to differences in the rates of transport into cells (e.g. the lighter molecules of ¹²CO₂ diffuse into photosynthetic cells more rapidly than ¹³CO₂) and of biochemical reactions within cells (which proceed faster with the lighter isotopes, and cause the kinetic isotope effect). The extent of isotopic fractionation is measured from the two most abundant stable isotopes and expressed as the ratio of the heavier over the lighter. However, because a single isotope dominates for all of the main elements of life, there can be very large errors in measuring isotope ratios in this way. To minimise errors, the ratio for a sample is usually expressed relative to a universal standard, as shown in the following formula for carbon:

δ¹³C (‰) = [{(¹³C/¹²C sample)/(¹³C/¹²C standard)} -1] x 1000

The ratios D/H (δD or δ²H),¹⁵N/¹⁴N (δ¹⁵N),¹⁸C/¹⁶O (δ¹⁸O) an δ³⁴S/³²S (δ³⁴S) are calculated in an analogous way, and all share units of parts-per-thousand, or permil (‰). In all cases negative values indicate that a sample is depleted in the heavier isotope relative to the standard, and it can be described as isotopically light. Conversely, samples with positive δ values are termed heavy. For carbon the standard is usually PDB (the Cretaceous Peedee Belemnite), for hydrogen it is SMOW (standard mean ocean water) and for sulfur CDT (Canyon Diablo troilite).

Click here for information on the techniques and standards used in isotope analysis.

Whether a δ value is positive or negative is less important than the relative change in δ when interpreting data:

• Isotopically heavier = enriched in the heavier isotope, resulting in a positive increase in values
• Isotopically lighter = enriched in the lighter isotope, resulting in a decrease in values

We are primarily concerned with the most frequently used isotopic ratios in petroleum geochemistry, those of carbon, hydrogen and sulfur. The factors governing isotopic fractionation are discussed by Hoefs (1997). Variations in stable isotope fractionation provide a potentially powerful tool for understanding biogeochemical processes.

Carbon and Hydrogen

Use can be made of both carbon and hydrogen stable isotope ratios when dealing with kerogen and petroleum because both elements are abundant. While significant strides have been taken in our understanding of factors affecting carbon isotope ratios, we are still relatively unenlightened about H isotope fraction in biogeochemical systems, and there are various technical problems when attempting to measure δD values. Consequently, most information to date has been obtained via C isotope ratios, but H isotope ratios are, nevertheless, used to obtain some source-related information and in correlation studies. Schoell (1984) presents a valuable summary of the application of C and H stable isotopes in petroleum geochemistry.

It is convenient to divide the detailed consideration of C and H isotope applications into:

• kerogen - indications of sources of organic matter and effects of hydrocarbon generation;
• oils - oil-oil and oil-source correlation;
• carbon gases - source and maturity evaluation, especially for methane.

Sulfur

Our understanding of sulfur isotope fractionation is improving, although the processes affecting the isotopic signature of kerogen (and hence oil) are complex. Sulfur isotope signatures can aid oil-source rock correlation and appear to reflect the amount of sulfate available to sulfate reducing bacteria during diagenesis. Hydrogen sulfide generation from kerogen is minor, and although some can come from bacterial sulfate reduction, most originates from thermochemical sulfate reduction, and isotope ratios can help identify the source.

Oxygen

One of the main uses of oxygen isotopic fractionation is in determining the volume of ice sheets from the δ¹⁸O values of benthic foraminifera. During periods of extensive glaciation the δ¹⁸O value decreases because H₂¹⁶O evaporates more readily from seawater and becomes locked up in ice sheets (evaporation can similarly affect C and H isotopic ratios in petroleum compounds). Once the glacial effect is known, estimates of sea surface temperature can be obtained from planktonic foraminifera. This application of δ¹⁸O values is beyond the scope of this manual.

Nitrogen

Nitrogen isotopes are rarely used in petroleum geochemistry although nitrogen gas may be present in significant volumes. Nitrogen is also present in kerogens and, to a lesser extent, in oils. However our understanding of the pathways of incorporation of the various nitrogen-containing substrates (nitrogen, nitrate, nitrite and ammonium) and associated kinetic fractionations into photosynthetic organisms is relatively sparse. Some useful references concerning nitrogen isotopes include:

• Ader et al. (1998)
• Littke et al. (1995)

Helium

The isotopic composition of helium is useful in determining potential deep crustal/mantle contributions to carbon dioxide/methane in gas seeps and accumulations.

References

Ader, M., Boudou, J.-P., Javoy, M. & Goffe, B. (1998). Isotope study on organic nitrogen of Westphalian anthracites from the Western Middle field of Pennsylvania (USA.) and from the Bramsche Massif (Germany). In: Organic Geochemistry vol. 29 pp. 315-325.

Hoefs, J. (1997). Stable isotope geochemistry., Springer-Verlag.

Littke, R., Kroos, B., Idiz, E. & Frielingsdorf, J. (1995). Molecular nitrogen in natural gas accumulations: generation from sedimentary organic matter at high temperatures. In: American Association of Petroleum Geologists Bulletin vol. 79 pp. 410-430.

Schoell, M. (1984). Stable isotopes in petroleum research. In: Advances in Organic Geochemistry , Academic Press Inc. pp. 215-246 ISBN: 0-12-032001-0.

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